Conductance quantization with \(\delta G = e^2/h\)  is expected in any QPC when the spin degenaracy is lifted by a magnetic field \(B\)  [1]}. However, even in the absence of \(B\) , such half-quantization has been reported in many studies of gated constrictions and quantum wires based on InAs [2]}, [3]}, [4]}, GaAs [5]}, [6]}, [7]}, and carbon nanotubes [8]}. The precise origins of this effect have arguably not been elucidated [3]}, [4]}. One ingredient suspected to be important is the presence of either intrinsic or field-induced spin-orbit interaction, resulting in spontaneous spin polarization [11]} or filtering transmission for opposite spins [12]}. Another possible essential ingredient is electron-electron interaction in 1D confinement, creating a spin incoherent [13]}, [14]} or helically ordered Luttinger liquid. Another proposed mechanism is disruption of the adiabaticity of the constriction-lead coupling by the disorder potential [4]}.
